Feline Lindhout

1 General introduction 15 ~80% plus-end and ~20% minus-end out microtubules (Yau et al. 2016). Thus, during the transition of unpolarized neurons (stage 2) to polarized neurons (stage 3), the microtubule cytoskeleton drastically reorganizes in both axons and dendrites, and new insights underlying this process are presented in Chapter 3 . The axonal ER network The ER is the largest organelle in neurons and other cell types. It is a highly dynamic structure that undergoes continuous remodeling, while the continuity of the structure remains intact. The ER network is composed of perinuclear ER sheets and peripheral ER tubules, which typically reflect rough and smooth ER, respectively. The rough ER contains ribosomes and is involved in protein translation. Instead, the smooth ER lacks ribosomes and does not contribute to protein translation, but acts as a key regulator of Ca 2+ homeostasis, signaling, and lipid synthesis and delivery. Axons primarily contain smooth ER, and the structure of axonal smooth ER differs significantly from other smooth ER structures (Yalcin et al. 2017; Wu et al. 2017; Terasaki 2018). The unique axonal ER structure is characterized by thin ER tubules at the shaft, with 1 or 2 tubules per axon diameter, and local ER cisternae at presynaptic boutons (Yalcin et al. 2017; Wu et al. 2017; Terasaki 2018). The importance of the axonal ER network is illustrated by axon degeneration diseases (i.e. hereditary spastic paraplegia and amyotrophic lateral sclerosis) caused by mutations in generic ER proteins (Hazan et al. 1999; Zhao et al. 2001; Nishimura et al. 2004; Zuchner et al. 2006; Montenegro et al. 2012; Esteves et al. 2014; Yalcin et al. 2017). This includes proteins that facilitate ER tubule formation (e.g. atlastin-1, reticulon-2, receptor expression-enhancing protein 1 (REEP1), receptor expression-enhancing protein 2 (REEP2)) or act as ER receptor (vesicle-associated membrane protein(VAMP)-associated protein B (VAPB)) (Hazan et al. 1999; Zhao et al. 2001; Nishimura et al. 2004; Zuchner et al. 2006; Montenegro et al. 2012; Esteves et al. 2014; Yalcin et al. 2017). All these identified ER proteins are highly evolutionary conserved and essential for ER formation and function throughout different cell types. It remains largely unclear why the dysfunction of these important ER proteins is specifically associated with axonopathies. Nevertheless, it implies that the unique axonal ER structures, which are highly adapted to the extreme axonal morphologies, are particularly susceptible for structural ER perturbations. Accordingly, emerging evidence is starting to elucidate the implication of local ER structure and dynamics in axon function (Ozturk, O’Kane, and Perez-Moreno 2020) ( Chapter 4 ). Role of centrosomes in developing neurons Transitions of neurodevelopmental stages, including axon formation, are accompanied with drastic changes in cell morphologies that are steered by large cytoskeletal remodeling (Neukirchen and Bradke 2011; Stiess and Bradke 2011; van Beuningen and Hoogenraad 2016; Schelski and Bradke 2017). A particularly important organizer of the cytoskeleton is the centrosome, the predominant microtubule-organizing center (MTOC) in cells. Although the precise role of centrosomes during specific neurodevelopmental processes is still